Digital systems receive and transmit data in the form of digital words that are defined as patterns of binary bits. Such systems may employ switches to generate for example, binary coded decimal words that correspond to selected decimal numbers.
Rotary-type switches have been used to select particular coded words from a preset list of words. However, rotary switches generally do not have the capacity to generate signals for relatively large numbers of words. Also, it is generally difficult to change the code of an existing word of a rotary switch or to add a word to the list of words of the switch.
It has been determined that a code word may be readily modified if each bit of the word is defined by a separate switch. Thus, the switches may be operated to define any possible combination of bits for the word. However, the word selection process is relatively slow and is prone to operator error, since more than one switch must be operated to define a word. Also, the switches which define a single word usually take up a relatively large amount of space on an instrument panel.
The disadvantages of the above-indicated switches have been overcome in part by a commercially available encoding switch which employs cam modules to define digital words. In operation, a cam module is engaged with the body of the switch so that cam surfaces of fingers or tabs of the module press flexible terminals of the switch against associated stationary terminals and thereby define the bits of a word. The bits of the word may be changed by removing the associated cam module and inserting a new module which has a different pattern of cam surfaces.
Each digital word of the known cam-operated encoding switch is defined by a corresponding cam module which is manufactured with a particular pattern of cam surfaces. As the number of output bits of a switch is increased, the number of different cam modules for the switch also increases and the complexity and expense of the cam manufacturing process is likewise increased, since additional unique modules must be produced. Also, spare modules must be purchased and stored for every word of a switch, in order to ensure that a replacement module will be available if any operating module is lost or damaged.
The cam modules of the prior art encoding switch are marked to indicate the identity of the associated code words. However, if an identifying mark of a module is removed or obliterated, it may be somewhat difficult to identify the module's code by inspecting its cam surfaces. Thus, the relatively complex activating fingers of known cam modules do not appear to be particularly suitable for use in a simple, compact and relatively low cost encoding switch.
Accordingly, it is an object of the invention to provide a compact encoding switch which generates a relatively large number of coded bits, for example eight binary bits.
A further object of the invention is to provide an encoding switch that utilizes a single uniformly shaped encoding module which may be easily modified by a user to define the bits of a particular code word.
Another object of the invention is to provide an encoding switch with an encoding module that has a physical structure that is clearly representative of the coding operation of the module.
A further object of the invention is to provide a blank encoding module with frangible tabs that are selectively removed to define a desired code word for an encoding switch.
It will be appreciated by those skilled in the art that an encoding switch will not generate coded output signals until power is applied to the switch. Thus, an encoding module may be inserted into the body of an unpowered switch and thereafter, power may be applied to the switch to generate corresponding output signals.
In practice, it is expected that the body of an encoding switch will typically remain "plugged" into a powered printed circuit board when encoding modules are exchanged in the switch. Accordingly, it is possible that a powered switch will generate momentary random signals when a module is initially inserted, since the module will probably engage switch terminals in a random order. Such random signals are generally undesirable since they may interfere with the proper operation of a circuit that monitors the switch.
Accordingly, it is an object of the invention to provide an encoding switch with a body portion that remains powered and encoding modules that may be inserted into the powered body to define bit patterns, without generating undesirable random intermittent signals.
A further object of the invention is to provide an encoding switch with an encoding module that initially establishes data connections which define a particular code pattern and after the connections are established, makes a common power connection for the switch.
A light source may be placed within an encoding switch to illuminate the switch when coded signals are being generated, so that an operator has a positive indication of the operational condition of the switch. However, the light source may generate heat within the switch and the heat may cause engaged terminals within the switch to expand and to move out of contact with one another.
Thus, it is an object of the invention to provide a lighted encoding switch with an encoding module which maintains the connections of terminals within the switch when the switch is internally heated by a light source.
It is known that the reliability of an encoding switch may be reduced if dust particles within the switch interfere with conductive connections between terminals of the switch.
Accordingly, a further object of the invention is to provide an encoding switch wherein reliable switch connections are achieved by wiping switch terminals together.